Just make sure to put the water back in the tank slowly. Also make sure that the hose is as far away from the water as possible when filling up. This would allow the the water to de gass and be less stressful for the fish.
I think this is actually a great idea. Whenever I am filling up with the python I am always wondering if particles of chlorine are getting to my fish. I have been doing it this way for years though without any problems. I think I worry too much..
